Arenes organic synthetic applications

Organometallic complexes often activate the C-H either by oxidative addition (Fig. 6.3, path a) or a-bond metathesis, or a-CAM (path b). These reactions favor attack at a terminal C-H bond, leading to subsequent terminal functionalization (e.g., PrH n-PrX), or at an arene C-H bond (e.g., ArH ArX). This selectivity usefully contrasts with standard organic reactions via radicals or carbonium ions that are selective for the most highly substituted or benzylic CH bonds (e.g., PrH i-PrX ArMe ArCH2X). Species such as i-Pr- or i-Pr+ are more stable and more rapidly formed than n-Pr- or n-Pr+. Numerous organic synthetic applications of C-H activation continue to be found (Chapter 14). 

Akira Suzuki, from Kurashiki University, Japan, is one of the great names of arene chemistry due to his outstanding reaction that bears his name. As expected, his chapter (No. 3) deals with the Suzuki reaction with arylboron compounds and its synthetic applications in arene chemistry. The Suzuki reaction is now becoming all the more important as requirements for organic synthesis will necessarily have to take ecological aspects ( green chemistry ) into account and use of a number of toxic metals is phased out. 

Polymer-supported (diacetoxyiodo)arenes have found broad synthetic application. Poly[ (diacetoxyiodo)styrene] (4) has been used for the oxidation of alcohols catalyzed by TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) [27-30], or in the presence of KBr [31]. In a specific example, primary alcohols are readily oxidized to methyl esters upon treatment with reagent 4 in the presence of KBr in the acidic aqueous methanol solution (Scheme 5.5) [31]. Likewise, organic sulfides are selectively oxidized to the respective sulfoxides by reagent 4 in water in the presence of KBr [32]. 

D. J. Ramon, M. Yus, New Methodologies Based on Arene-Catalyzed Lithiation Reactions and Their Application to Synthetic Organic Chemistry, Eur. J. Org. Chem. 2000, 225-237. 

The oxidative activation of arenes is a powerful and versatile synthetic tactic that enables dearomatization to give useful synthons. Central to this chemistry are hydroxylated arenes or arenols, the phenolic functions of which can be exploited to facilitate the dearomatizing process by two-electron oxidation. Suitably substituted arenols can hence be converted, with the help of oxygen- or carbon-based nucleophiles, into ortho-quinone monoketals and ortho-quinols. These 6-oxocyclohexa-2,4-dienones are ideally functionalized for the construction of many complex and polyoxygenated natural product architectures. Today, the inherent and multiple reactivity of arenol-derived ortho-quinone monoketals and ortho-quinols species is finding numerous and, in many cases, biomimetic applications in modern organic synthesis. 

Although a wide range of Ti -arene transition metal complexes are known/ it is only ri -arene chromium tricarbonyl complexes that have found significant applications in synthetic organic chemistryThese complexes have been investigated either as key intermediates in synthesis " or as efficient catalysts in a variety of processes. 

Transition metal boryl complexes have been the subject of considerable recent research effort. These compounds find application as synthetically useful reagents in organic transformations such as hydroboration and diboration of carbon-carbon multiple bonds.More recently, the involvement of cyclopentadienyl transition metal boryl complexes in both stoichiometric and catalytic functionalization of alkanes and arenes has been demonstrated, notably by Hartwig ef Complexes CpFe(CO)2(Bcat) (cat = 02C6H4) 80, and GpFe(GO)2(BPh2), which have been... 

The major area of (arene)Cr(CO)3 chemistry however remains applications to synthetic organic chemistry. A study of nucleophilic and electrophilic substitution reactions of conformationally restricted (arene)Cr(CO)3 complexes has shown that nucleophiles react at carbons eclipsed by Cr-CO bonds and electrophiles at staggered postions.Both the stabilisation of a-carbocations and reactivity of a-hydrogens towards base has been investigated. The substitution of arenes with alkyllithium reagents has been studied and measurements of acidities carrried out. ... 

As noted in the Preface to Volume I, the second volume includes chapters on the applications of arene and alkyne complexes, as well as cluster compounds, in organic synthesis. Other useful synthetic transformations are discussed in the last chapter. A chapter on insertion reactions of synthetic utility was not included, due to the publication in the last few years of a substantial number of fine reviews in this area. 

---